Intramolecular azavinyl carbene-triggered rearrangement of furans

Rhodium-Catalyzed Double Dearomatization of 1,2,3-Triazole-Isoxazole Dyads: Synthesis of Nonfused 1H-1,3-Diazepines

A double dearomatization of dyads consisting of 1-sulfonyl-1,2,3-triazoles and 3-aryl-5-methoxyisoxazoles was applied for the efficient synthesis of nonfused 1H-1,3-diazepines. The plausible mechanism of the cascade reaction includes transformation of the 1,2,3-triazole to rhodium azavinyl carbene, the Z-selective hydride shift to form the 1-azabuta-1,3-diene moiety, rhodium-catalyzed ring contraction of the isoxazole to azirine, and pseudopericyclic four-atom ring expansion of the azirine. The synthetic utility and antiproliferative activity of the 1,3-diazepines obtained have been demonstrated.

Axial Ligand Enables Synthesis of Allenylsilane through Dirhodium(II) Catalysis

Described herein is a dirhodium(II)-catalyzed silylation of propargyl esters with hydrosilanes, using tertiary amines as axial ligands. By adopting this strategy, a range of versatile and useful allenylsilanes can be achieved with good yields. This reaction not only represents a SN2'-type silylation of the propargyl derivatives bearing a terminal alkyne moiety to synthesize allenylsilanes from simple hydrosilanes, but also represents a new application of dirhodium(II) complexes in catalytic transformation of carbon-carbon triple bond. The highly functionalized allenylsilanes that are produced can be transformed into a series of synthetically useful organic molecules. In this reaction, an intriguing ON-OFF effect of the amine ligand was observed. The reaction almost did not occur (OFF) without addition of Lewis base amine ligand. However, the reaction took place smoothly (ON) after addition of only catalytic amount of amine ligand. Detailed mechanistic studies and density functional theory (DFT) calculations indicate that the reactivity can be delicately improved by the use of tertiary amine. The fine-tuning effect of the tertiary amine is crucial in the formation of the Rh-Si species via a concerted metalation deprotonation (CMD) mechanism and facilitating β-oxygen elimination.

Total Synthesis of (-)-Rauvomine B via a Strain-Promoted Intramolecular Cyclopropanation

We describe the first total synthesis of the unusual cyclopropane-containing indole alkaloid (-)-rauvomine B via a strategy centered upon intramolecular cyclopropanation of a tetracyclic N-sulfonyltriazole. Preparation of this precursor evolved through two generations of synthesis, with the ultimately successful route involving a palladium-catalyzed stereospecific allylic amination, a cis-selective Pictet-Spengler reaction, and ring-closing metathesis as important bond-forming reactions. The key cyclopropanation step was found to be highly dependent on the structure and conformational strain of the indoloquinolizidine N-sulfonyltriazole precursor, the origins of which are explored computationally through DFT studies. Overall, our synthesis proceeds in 11 total steps and 2.4% yield from commercial materials.

Unveiling Orthogonal Reactivity of Substituted 2-(2-Azidostyryl)furans: Thermolysis and Photolysis versus Catalysis

A systematic investigation of the decomposition of substituted 2-(2-azidostyryl)furans has been reported. The products of catalytic decomposition align with predictable patterns, consistent with established literature data. In contrast, photolysis and thermolysis lead to the formation of unexpected products. In this case, generated nitrenes surprisingly exhibited an affinity for the furan core, deviating from the anticipated attack on the olefin moiety.

Interrupted Furan-Yne Cyclization: Access to Unsaturated Dicarbonyl Compounds and Their Subsequent Transformation into Functionalized Pyridazines

The key carbenoid intermediate of transition-metal-catalyzed furan-yne cyclization in Hashmi phenol synthesis could be efficiently intercepted with water under the developed reaction conditions in order to provide access to functionalized unsaturated dicarbonyl compounds that might serve as convenient precursors for the straightforward synthesis of annulated pyridazines.

Unearthing the Subtleties of Rhodium(II)-Catalyzed Carbenoid Cycloadditions to Furans with an N-Sulfonyl-1,2,3-triazole Probe

The rhodium(II)-catalyzed reaction of a model alkenyl donor/acceptor N-sulfonyltriazole with a wide selection of furans is reported. This investigation unearthed a range of structurally diverse carbocyclic and ring-opened products, in good to excellent yields. The products obtained are proposed to arise selectively via cyclopropanation or zwitterionic rearrangement pathways, which are highly dependent on both the structural and electronic features of the furan substrate.

Synthesis and reactivity of 1-sulfonylcyclooctatriazoles

Sulfonyl azides undergo rapid inverse electron demand SPAAC with strained alkynes to deliver 1-sulfonyl-1,2,3-triazoles. Treatment of these with Rh(ii) carboxylate catalyst promotes denitrogenation and transannular 1,5-H insertion or 1,2-H shift.

Acid-catalyzed ring-expansion of 4-(1-hydroxycyclobutyl)-1,2,3-triazoles

A direct ring-opening/semipinacol rearrangement reaction catalyzed by the Brønsted acid of 4-(1-hydroxycyclobutyl)-1,2,3-triazole is reported, in which the usage of metal catalysts and the reservation of sulfonyl groups are avoided.

Transformation of 3-(Furan-2-yl)-1,3-di(het)arylpropan-1-ones to Prop-2-en-1-ones via Oxidative Furan Dearomatization/2-Ene-1,4,7-triones Cyclization

The approach to 3-(furan-2-yl)-1,3-di(het)arylprop-2-en-1-ones based on the oxidative dearomatization of 3-(furan-2-yl)-1,3-di(het)arylpropan-1-ones followed by an unusual cyclization of the formed di(het)aryl-substituted 2-ene-1,4,7-triones has been developed. The cyclization step is related to the Paal-Knorr synthesis, but the furan ring formation is accompanied in this case by a formal shift of the double bond through the formation of a fully conjugated 4,7-hydroxy-2,4,6-trien-1-one system or its surrogate.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Intramolecular iron-catalyzed transannulation of furans with O-acetyl oximes: synthesis of functionalized pyrroles

Intramolecular iron(iii)-catalyzed reaction of furyl-tethered O-acetyl oximes yields substituted pyrroles through electrophilic 5-exo-trig ipso-addition of nitrogen to the furan ring.